Its the metadata for that book. I believe it's created so if you want to import those epubs into a different program, you can do so with all your nice metadata intact.

You can disable this behaviour if you want. Under Prefererences-->Add/Save-->(TAB)Saving Books. There is a checkbox for "Save Metadata in OPF file".

EDIT: At least for epubs, I believe the metadata is also stored within the actual epub, so FOR EPUBS this OPF file might be redundant. But not all ebook formats support saving extensive metadata inline (think TXT, RTF, HTML, etc). I could be wrong about this though, so someone else pipe in.

Its the metadata for that book. I believe it's created so if you want to import those epubs into a different program, you can do so with all your nice metadata intact.

It's also used when adding the book to Calibre. If that file exists when you try to import into Calibre, it will override any other metadata in the file or in the filename. If you want to give a book to a friend who's also using Calibre, give them the opf file too.

Quote:

At least for epubs, I believe the metadata is also stored within the actual epub

Calibre stores an opf file with every book in its library, changing it every time the metadata for the book is changed. The files are used by the restore database function to recreate calibre's database (metadata.db) in the case that the db has become corrupted.

You cannot turn off this behavior. You can, of course, delete the files, but by doing so you are potentially shooting yourself in the foot by depriving yourself of a way to recover your library if something goes wrong with the db.